All you need to know about charging these old calculators is the shape and polarity of the plug and the number/size of the nicad cells. Almost all of them use AA, so figure that back then they were around 700mAH per cell and charge at a 1/10 rate. Voltage isn't important, just enough to give you current needed, 70mA in this case. Of course almost no vendor used a regulated charger back then so the 70mA should be the average during the charging cycle -- it will be more at the start and less at the end.

Back when I used to charge my rechargeable calculators, I would experimentally pick a charger from a box of junk ones and measure the current when connected to the numbers of cells in the calculator. When I found the right one I'd solder on a plug and that was my custom-built charger. This worked great until I tired of charging them every few months. That's when a moved to primary AAA cells in plastic battery holders to replace AA nicads. Additionally, I use Energizer lithium AAA cells since they have near zero self discharge and never seem to leak.

There is a special ARISTO charger on the market - well, I should say it was, but you find it on a German auction site every once and a while. And yes, ARISTO got awards for "good industrial design" quite regularily those days. You know why :-)

Why do not use ordinary batteries 3 x 1.5 AA?
Specs:4.5v DC using 3 x AA size batteries.
It accepts an adaptor (type 6795) through a socket on the top side to the far left.
You have to take the whole back off to access the three (2 plus 1 separated) battery slots, one of which is not used.

The old AC nicad chargers were usually not regulated, most new universal ones are. So a 7.5 volt setting will be 7.5 volts all the time, that's almost certainly going to pump way too much current into the 3 x AA nicads and they will get hot and be damaged.

The old AC chargers were essentially current limited devices due to their small transformer size, that's what kept the nicads from overheating and being damaged. The voltage rating on them was just a nominal value for "voltage at the rated load" if they had a rated load. If they had no rated load the voltage was pretty much meaningless, open circuit these would often be twice the stated value. There's no substitute for actually measuring the current draw when trying to fit a charger to a calculator.

The ideal solution is to get an external power supply that let's you limit the current and monitor it. Then you'll really know what's going on and have a single charger for all your calculators.
For not much money you can find ones that are perfect for almost any battery powered calculator:

The LED version of the Aristo M85 is designed to be operated with 3xAA Alkaline or NiCd cells or with a 7.5V= max.200mA voltage supply (Aristo #6790) to save batteries. I have just verified that minus is on center.

The M85 is NOT designed to recharge batteries inside the calculator, so please do NOT try to connect it to a current source. Aristo sold an external recharger (#6795) for NiCd cells. Actually, the M85 runs quite long on a single set of Alkalines, so you'd be best off just using these.

The "M85 S" model has a builtin NiCd battery. There was a later version of the M85 with a green fluorescent display operating on 2xAA cells.

This is one of the most beautiful and mechanically best calculators ever built. The keys have no click, but excellent response. They precisely register at approx. 1/3 of their 2mm travel without bouncing ever, even after 40 years of (initially intense, later occasional) use.

IIRC, its LEDs were manufactured by HP. Arithmetic is chain logic (i.e. no precedence, two levels of parentheses). Log, exponential and trigonometric functions are computed to six digits.